Banknote and security papers available in the marketplace today may incorporate a wide variety of materials to assist in verifying the authenticity of a document. The concept of incorporating optically variable devices OVD's as security elements in paper has received considerable attention in recent years.
As described in European Pat. No. 0059056, an optically variable dichroic device may be incorporated in the form of a continuous ribbon running through the material of the sheet. The elongated element, similar to the commonly used security thread, would be made visible at the surface at spaced locations during manufacture.
British Patent Specification No. 1552853, describes a dichroic filter material in the form of an elongated element formed into the substance of the sheet in a method similar to commonly used security threads. In this patent, the element is made visible through apertures or superposed windows in the base sheet. This process however requires the removal of overlaying fibers by some mechanical action imparted after the papermaking process.
In another British Patent Specification No. 1,365,876, a porous tape is described which can be formed within the substance of banknote paper. This tape consisting of a variety of specialized materials is incorporated within the banknote to yield unique properties. The advantage of the porous tape compared to the widely used, impervious, thin film, security thread is that it can be incorporated in far greater widths owing to its porous nature.
The deficiency of a continuous security strip, or ribbon having optically active properties lies in the narrow width to which it is confined. A constraint of approximately 2 mm maximum width is imposed by the sheet-forming process. Strips any wider than this cannot be reliably formed within the substance of the paper with a uniform fiber layer on either side. In addition, the location at which the elongated strips are exposed cannot be registered to specific locations during the papermaking process. Therefore, the entire security strip must be made optically active. Patterning within the optical strip cannot rely on being exposed in a registered format. Since it is important that significant portions of the strip be trapped within the fibers of the sheet to prevent removal, only small, narrow segments remain visible at the surface of the document. The limited refracting power of such small exposed areas severely limits its usefulness.
While a porous tape can be used to incorporate a wider elongated strip, there is no known method to date for exposing such a wide strip at the surface of the paper. With overlaying paper fibers at least partially obscurring the strip its ability to refract unscattered light having distinct color separation is limited.
The method of making windows or apertures disclosed in British Patent Specification No. 1,552,853 involves off-machine processing of finished paper. Since this process depends on removal of fibers, localized weakening of the paper can often occur.
An efficient method for incorporating security designs in paper is described within U.S. Pat. No. 4,437,935 to Fred Crane, which patent is incorporated herein for purposes of reference. This patent teaches the attachment of an optical element to a carrier web and application of the carrier web to a base web during the dewatering process within a Standard Fourdrenier paper making machine. However, to insure adequate dispersion of the base web fibers into the carrier web, the process is carried out at a controllable rate much slower than a conventional paper making process. It has since been determined that by modifying the paper making machine to include an embedment roll, the security paper can be manufactured in the same time required to make ordinary high quality banknote paper.